The present invention relates to an optical disk apparatus for optically recording and reproducing information and, more particularly, to a track access control method in which a plurality of beam irradiation lenses (objective lenses) are arranged in series with each other in a recording/reproducing direction, and information is recorded at a high speed by erasing/recording information with a preceding beam and by recording/reproducing information with a following beam, and an optical disk apparatus using this method.
In an optical disk apparatus for optically writing and reading information in and from a disk-like recording carrier, and particularly a magneto-optical disk apparatus having an information rewrite function, two beam irradiation cycles, i.e., a cycle for irradiating a light beam to erase information, and a cycle for irradiating a modulated light beam for recording new information are required.
To eliminate a cumbersome recording procedure and shorten the processing time in such a magneto-optical disk apparatus, an optical disk or a magneto-optical disk apparatus separately using an information erasing beam and an information recording/reading beam has been proposed.
For example, an arrangement in which two light beams are emitted through one objective lens to use one for erasing information and the other for recording/reading information is disclosed in Japanese Patent Laid-Open No. 61-250846.
Although the arrangement in which the erasing beam and the recording beam are close to each other is effective for a phase change type rewritable optical disk which requires no external magnetic field for recording and erasing, this arrangement is difficult to be applied to the magneto-optical disk apparatus. In the magneto-optical disk apparatus in which recording and erasing magnetic fields must be inverted, positions where the recording and erasing magnetic fields applied to a medium must be spaced apart from each other by a certain distance. It is preferable to irradiate beams focused by corresponding objective lenses on erasing and recording positions, respectively.
A magneto-optical disk apparatus having such light beams is disclosed in, e.g., Japanese Patent Laid-Open No. 3-263638. In this example, a head for forming a spot for recording/erasing information, and a head for forming a spot for reproducing information are mounted on the same carriage, and the erasing/recording spot precedes the reproducing spot on the same track by a predetermined distance.
In the optical disk apparatus in which the separate optical heads or objective lenses are arranged in order to irradiate two beams on positions relatively apart from each other on a medium, and are mounted on the same carriage to move the beams in the radial direction of the disk (direction perpendicular to a track), the relative positions of the two beams must be kept almost constant with respect to a track on the disk. Particularly, in recording and reading operations, the two beams must be present on the same track.
To allow the two beams to rapidly reach the same track upon completion of a track access, the two beams desirably move following almost the same trace without greatly shifting the relative positions of the two beams during the inter-track movement of the beams (track access).
On the other hand, a preferable track access control method for the apparatus having the two objective lenses or optical heads mounted on the same carriage has not been attained.
One of easily assumed methods is a method of stopping (activating a stop servo) the objective lenses with respect to the reference axis of a carriage or head in a track access to move the carriage with reference to a track position signal detected from either beam.
In this method, however, the relative positional precision of the two beams during the track access depends on the positional precision of the lens stop servo and the servo characteristics, and a precision enough to position the beams on the same track cannot be expected. Therefore, at the end of the carriage movement, the positions of the two beams are greatly shifted from each other. To align the two beams on a target track, the beams must be precisely sought (track jump seek), requiring a long period of time.
In the optical disk apparatus in which a plurality of objective lenses are mounted on the same carriage to simultaneously perform erasing and recording operations, the time required for a track access in which beams are moved to a target track is greatly prolonged.